What Reason Does Macbeth Give The Murderers To Kill Banquo

Macbeth tells the murderers that he has his own reasons for
wanting Banquo murdered, and he provides them with several reasons
why they should do it before he challenges their masculinity (a
quick study--he's learned from the master--oops mistress).

We aren't told precisely what Macbeth has done, but that he has done something can be inferred from his comments to the murderers, all of whom apparently have a grudge not against Banquo, but against Macbeth....

Macbeth tells the murderers that he has his own reasons for
wanting Banquo murdered, and he provides them with several reasons
why they should do it before he challenges their masculinity (a
quick study--he's learned from the master--oops mistress).

We aren't told precisely what Macbeth has done, but that he has
done something can be inferred from his comments to the murderers,
all of whom apparently have a grudge not against Banquo, but
against Macbeth. Macbeth tells them that it was Banquo and not his
"innocent self" that caused their undoing. In fact, he tells
them that he has already informed them of exactly how and by what
means the offensive deeds were done. According to Macbeth, Banquo
is the one whose "heavy hand hath bowed [them] to the grave and
beggared them forever" (by which we understand that Macbeth must
have destroyed their lives and their livelihood so completely that
they are all desperate men.

Manhood is an important theme in the play. Indeed, Lady
Macbeth uses this as a way to prod Macbeth forward to kill the
King. Macbeth plays this card with the murderers when he asks
them if they can endure a man like Banquo, whose "heavy hand hath
bow'd you to the grave / and beggar'd yours for ever." Their
response: "We are men, my liege"(3.1.90-91). Macbeth then
tells them: "Both of you /Know Banquo was your enemy,"
explaining he himself cannot do the dirty work of killing Banquo
because of mutual friends, and for that reason "to [their]
assistance" does he "make love." To prove their manhood, and
because they have nothing to lose (see 3.1.108-114), they
agree.

IN Act III scene I Macbeth tells elludes to a previous
conversation he has had with them, yet which does not actually
appear in the text itself. Based upon what Macbeth says in
III.i, we are left to infer that he told the murderers that Banquo
was their enemy who wished upon their death and sought to do them
harm and ill will. The details of this are pretty scarce, but
Macbeth asks the murderers if their patience will be enough to
tolerate this, to which they say no. Later in the Act, an
anonymous, carefully disguised third murderer joins in - this third
murderer is probably there to make sure the murders follow through
on the murders of Banquo and Fleance.